Wikipedia has a great description about XML. Here is a quote about it:

XML (Extensible Markup Language) is a set of rules for encoding documents in both human-readable and machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards.

Extensible - XML is extensible. It lets you define your own tags.

Markup - The most attractive feature of XML has to be its ability to allow its user to create their own tags. The tags that can be created will be similar to tags in HTML. But with XML you are afforded the opportunity to define your own tags.

Language - XML is a language that is very similar to HTML. But it’s much more flexible because it allows to create custom tags. In this way XML acts like a meta-language: a language that allows us to create or define other languages. For example, with XML we can create other languages, such as RSS.

Another site is StackOverflow, with questions using tag XML.